torque?

[zhopalu]

hi, i want to know why do certain gas turbine engines express torque as a percentage and why does this torque vary with altitude & temperature?

 

[ppragman]

hi, i want to know why do certain gas turbine engines express torque as a percentage and why does this torque vary with altitude & temperature?

Zhopalu? You don't speak russian by chance do you?

 

[jynxyjoe]

I suppose at some point saying thousands upon thousands of lbs of torque gets a little silly.

Beech guys help out. PT6A-67D was 3950lbs takeoff and 3750lbs climb/cruise? CT7-9B we use % probably because at some point you are talking about too many lbs to make it worthwhile .

As far as altitude and temp, the higher (hotter) you go the less air available to burn. Going much deeper than that really requires a book or a long long long reply.

Hope that helps.

 

[jaded]

Probably because it's easier to read something as a percentage, with 100% torque being the most the propeller shaft can take during normal operations. Now as far as temperature altitude, remember that you really have 2 shafts spinning in that engine... the gas generator Ng and the propeller shaft Np. Since the air is more dense at sea level, the Ng doesn't have to spin too fast to put a lot of torque on that propeller shaft. For example in the winter time a takeoff at sea level your limiting factor will be your torque, you can have a torque of 100% during the takeoff with an ITT of let's say 800C. Now as your get higher in altitude, your torque decreases due to the less dense air (similar to how the manifold pressure decreases on a recip complex) you can adjust and increase the torque using the power lever thus increasing your ITT as well, until you get to a limiting number like say 860C (lets say thats what the book or company limits your ITT to) at that point, you can't increase the power anymore so as you climb your torque will keep decreasing, you just have to live with it.

 

[fish314]

I suppose at some point saying thousands upon thousands of lbs of torque gets a little silly.

Beech guys help out. PT6A-67D was 3950lbs takeoff and 3750lbs climb/cruise? CT7-9B we use % probably because at some point you are talking about too many lbs to make it worthwhile .

As far as altitude and temp, the higher (hotter) you go the less air available to burn. Going much deeper than that really requires a book or a long long long reply.

Hope that helps.

I suppose another reason is that percentages give you a "nicer" point of reference from a human factors perspective. Would you rather look down at takeoff and see 100% or 3950 ft-lbs.? Suppose you saw 3937 ft-lbs? Would you be able to do the math yourself to see that this is still 100% (99.670886%, actually, but it rounds to 100).

 

[jynxyjoe]

I suppose another reason is that percentages give you a "nicer" point of reference from a human factors perspective. Would you rather look down at takeoff and see 100% or 3950 ft-lbs.? Suppose you saw 3937 ft-lbs? Would you be able to do the math yourself to see that this is still 100% (99.670886%, actually, but it rounds to 100).

Yeah. 100% is a lot easier. Though, on the beech, the colors on the analog dial made it pretty easy to set the torque. Where's a HF guy when u need one. Probably trying to color code my armrest so I know which one is for which arm.

 

[zhopalu]

thanx everyone. jaded, just an extension of the question, when we say a certain percentage torque, lets say 90%, then it is a percentage of what exactly? the maximum power that the engine can deliver to the shaft under the given atmospheric conditions?

 

[minitour]

90% of the rated power output.

-mini